Process for improving the hydrophilic properties on polymer surfaces

ABSTRACT

Polymer material is surface modified with the intention of increasing the hydrophilicity of the polymer surface, by producing carboxyl groups, carbonyl groups and hydroxyl groups on the polymer surface in a first stage, by oxidizing with, e.g., oxidizing acid solutions or in some other manner, and by reacting the groups on the polymer surface in a second step with heterocyclic compounds having three or four ring atoms or with isocyanate compounds or carbodiimide compounds.

This application is a continuation of application Ser. No. 07/833,828filed Feb. 13, 1993 now abandoned. which is a continuation ofapplication Ser. No. 07/573,147 filed Sep. 19, 1990 now abandoned.

The present invention relates to a method for modifying the surfaces ofpolymeric materials. More specifically, the method is intended toincrease the surface energy of polymer surfaces and therewith renderhydrophobic polymeric materials hydrophilic.

An increase in the surface energy and hydrophilicity of synthetic resinsurfaces and fiber surfaces comprising hydrophobic polymeric materialsis a highly significant factor in the improvement of the properties ofsuch materials with respect to gluing, adhesion, painting, dying,metallizing and printing.

Improvements in these properties have been decisive in meeting theexpanding usage of polymeric materials in practical applications.Hitherto, the hydrophilicity of hydrophobic polymers has been increasedby the following methods:

1. Oxidation with the aid of strongly oxidizing solutions.

2. Oxidation with the aid of physical-chemical methods, such as corona,flame and plasma treatment.

3. Grafting with hydrophilic monomers.

Known chemical oxidation methods may include the use of solutions ofchromium oxides, permanganates and perchlorates in strong mineral acids.These chemical methods have resulted in a very marginal lowering of thecontact angle used as a measurement of the surface energy of polymersurfaces. Furthermore, it is difficult to obtain reproduceable resultson the contact angle when applying these methods. Most polymers have alow surface energy (19.1 mJM⁻² in the case of polytetrafluoroethylene;30 to 33 in the case of polyethylene, and 47. mjm⁻² in the case ofpolyethylene terephthalate) which means that they exhibit a high contactangle at the air/water interface, in the order of 80°-95°, and cannottherefore be wetted readily with water.

A further drawback with the earlier published treatment methods is thatthat treatment is not stable and that the contact angle returns to itsoriginal values during the storage or use of such treated materials.Consequently, the effect of the treatment disappears with time andcannot therefore be used beneficially in practical contexts.

The improvement achieved in the hydrophilicity of polymer surfaces bythe oxidative treatment of these surfaces is mainly due to theoccurrence of polar groups on the polymer surfaces, mainly carboxyl,hydroxyl and carbonyl groups. The physical-chemical methods recitedunder point 2 above will generally result in the formation of similargroups on the polymer surfaces, and the limited success of these methodsis due to the same reasons, namely an insufficient increase in surfaceenergy and poor stability with regard to time.

The grafting methods referred to in point 3 above include a large numberof known methods which although resulting generally in stablehydrophilicity, are expensive because of the complicated processapparatus required and the considerable monomer and catalyst inputentailed. An additional drawback with graft copolymerization methodsresides in the difficulties experienced in removing the simultaneouslyforming homopolymers, and in the very long extraction times requiredherefor.

The object of the present invention is to provide a method which willincrease the surface energy and hydrophilicity of polymer surfaces,which will result in permanent hydrophilicity, and which because of theshort treatment times and low chemical consumption entailed is suitablefor industrial applications.

Accordingly this invention consists in a method which is mainlycharacterized by subjecting to oxidation in a first step, polymericmaterials which include polyolefins, polystyrene, fluoroplastics,polyesters, polyacetals and polyacrylates, such as to produce on thepolymer surfaces functional groups such as carboxyl, hydroxyl andcarbonyl groups with the aid of known methods chosen from etching withoxidizing acid solutions, corona-treatment, plasma treatment andflashing treatment, and said first step also including the selection ofpolymerisation conditions in the manufacture of the polymeric materialsuch that said functional groups will be incorporated in the polymerchains, and by bringing the resultant polymer surface in a second stepinto liquid contact with organic compounds which include heterocycliccompounds having three or four ring atoms, such as oxiranes, thiiranes,aziridines, azetidinones, oxetanes carbodiimides and isocyanates.

A fundamental concept of the development work lying behind thedevelopment of the inventive method is that the from a steric point ofview readily mobile functional groups obtained with the known oxidationand etching processes will, in time, rotate inwardly in the bulk of thepolymer, therewith explaining the subsequence diminishment ofhydrophilicity, Accordingly, the next step in this development work wasto find a suitable reagent which would bind these functional groups tonew, larger groups of lower steric mobility. The choice of reagent opensfurther possibilities of increasing and regulating the extent to whichthe polymer surface is hydrophilized.

The reagents chosen for the inventive method step fulfill therequirements of being highly reactive, and therewith having shortreaction times, and provide sterically stable groups of elevatedhydrophilicity, in comparison with the non-reacted groups obtained withan oxidation treatment process.

Chemical oxidation of a polymer surface in accordance with step 1 can beeffected with known, standard etching solutions, such as chromiumtrioxide, or a dichromate solution in water, or sulphuric acid, chromiumtrioxide dissolved in phosphoric acid and aqueous sulphuric acid,permanganate solution in sulphuric acid, peroxodisulphate solution inwater, chlorate solution in perchloric acid, or by first etching with achromium trioxide solution in sulphuric acid, followed by etching withconcentrated nitric acid. The time taken to complete the etching processcan vary between 5 seconds and 30 minutes and the process temperaturemay vary from room temperature to 100° C.

The choice of the etching method employed is, to some extent, related tothe polymer to be treated, Olefin plastics are relatively inert and arepreferably treated with a dichromate solution in sulphuric acid.Polystyrene, on the other hand, is liable to degrade in stronglyoxidizing acid solutions, and consequently milder oxidizing processesshould be employed. A suitable polystyrene oxidizing method is one inwhich polystyrene is treated with hydrogen peroxide in the presence ofUV-light. Fluoroplastics are among those plastics which are leastreactive chemically and therefore demand very harsh etching conditions,e.g. treatment with a solution of alkali metal in liquid ammonia.

Polyesters and polyacrylates are also liable to degrade in oxidizingacid solutions. However, an alternative method to step 1 is hereproposed. Since polyesters are produced by the co-condensation ofpolyols and dicarboxy- lic acids, it is possible to obtain an excess ofcarboxyl and/or hydroxyl groups by controlled stoichiometric imbalanceof the reactive components during co-condensation.

When producing polyacrylates, carboxyl and/or hydroxyl groups can beformed in the finished polymer, by adding monomers which containhydroxyl and/or carboxyl groups to the polymerisation process. In otherwords, step 1 is carried out during the actual polymerisation process.

Oxidative treatment in step 1 using plasma, corona or flashingtreatments can be effected on all types of polymer embraced by theinvention. When plasma-etching, the material is exposed to a plasmawhich comprises a mixture of reactive substances, such as atoms,molecules and ions, in a metastable and/or excited state, and electrons.The mixture ratio of these reactive substances is such that a totalbalance prevails between positively and negatively charged particles.

Apparatus for corona or flashing treatment are available commercially,and these processes must be considered to be well established techniquesin the present context.

In step 2, the polymer treated in accordance with step 1 is brought intocontact with a solution which contains a compound belonging to thefollowing groups A and B, wherein group A includes heterocycliccompounds having three or four ring atoms, such as oxiranes, thiiranes,aziridines, azetidinones and oxetanes, and group B includescarbodiimides and isocyanates. More specifically, the groups include thefollowing compounds:

AI. Oxiranes ##STR1## where R₁ is --CH₂ Hal or CH₂ OH and Hal is Cl, Bror I,

AII: Thiirane of the formula ##STR2##

AIII: Mono- or multifunctional aziridenes of the formula ##STR3## whereR' may be hydrogen or an alkyl group having 1-10 carbon atoms and R₂ maybe an alkyl group having 1-10 carbon atoms or an alkyl group having 1-10carbon atoms which is substituted with one or two further aziridinegroups, a melamine group in which one, two or three hydrogen atomscoupled to carbon atoms in the melamine ring are substituted with one ormore aziridine groups or a P═O group.

AIV: Azetidinones of the formula ##STR4## in which R₃ and R₄ includehydrogen, an alkyl group having 1-3 carbon atoms or an hydroxyl group.

AV: Oxetanes of the formula ##STR5## in which R₅, R₆ and R₇ includehydrogen, an alkyl group or an hydroxyl group.

BI: Isocyanates of the formula

R₈ --N═C═O, in which R₈ is an alkyl group having 1-10 carbon atoms, andtoluene diisocyanate, alkylene diphenyl diisocyanates, isophoronediisocyanate, xylene diisocyanate and alkylenedicyclohexyl-diisocyanates.

BII: Carbodiimides of the formula

R₉ --N═C═N--R₁₀, in which R₉ and R₁₀ are each an alkyl group having 1-10carbon atoms or a phenyl group which can be substituted with a halogenor hydroxyl.

The reaction according to step 2 is carried out in aprotic organicsolvents, such as ketones and ethers. The treatment time is generallyshort and is from between 30 seconds and 30 minutes, preferably between30 seconds and 3 minutes.

The temperature can be varied between room temperature and 100° C.,preferably between room temperature and 60° C.

The reaction in step 2 between the polymers and compounds in group Atakes place through ring-openings of the heterocyclic ring. Examples ofsuch reactions are: ##STR6##

Examples of reactions with compounds in group B are: ##STR7##

According to one preferred embodiment of the invention, the polymerobtained in step 1 is reacted in step 2 with a polyfunctional compound,i.e. a multi-ring heterocyclic compound chosen from group A, or, forinstance, with a diisocyanate group from group B. The followingcompounds can be mentioned by way of example: ##STR8##

In these instances, the non-reacted functional groups remaining on thepolymer surface can be reacted in a third step with alcohols, polyols,monocarboxylic acids, or dicarboxylic acids for from 1 to 30 minutes attemperatures between room temperature and 100° C. Such unreacted groupswill occur on the polymer surface when a polyfunctional compound is usedto modify the polymer surface in step 2 above instead of amonofunctional compound. Since the concentration of --COOH, --OH andC═O-groups on the polymer surface is very low, it is probable that onlyone reactive group of the polyfunctional compound reacts with thesegroups, therewith leaving the other reactive groups for furtherreaction. This enables the polymer surface to be modified to variousdegrees of hydrophilicity.

In order to confirm that the presence of groups such as --COOH, --OH orC═O is necessary in order to hydrophilisize by reaction with thereactive compounds, all of the aforesaid polymer materials were treatedwith the various compounds described in step 2, without treating thepolymer surface in accordance with step 1.

Measurement of the contact angles showed no changes in surface energy.The invention is further illustrated below with the aid of a number ofworking examples.

EXAMPLE 1

The contact angle on untreated polypropylene film was measured, afterwashing the film with acetone and drying it was found to be 91°-95°. Thefilm was then treated with a dichromate solution in sulphuric acid (4.4%by weight dichromate in 15.7 molar of sulphuric acid) over periods of 30sec, 1 min and 5 min at 20° C., and periods of 5 sec, 10 sec, 1 min and5 min at 70° C., whereafter the film was washed clean with distilledwater.

The contact angle at the air/water interface of the treated films wasthen measured, subsequent to having dried the films in an oven at 50° C.for three hours. The measured contact angles had values of 90±2°; 901,7°; and 94.6±2.2° at 20° C., and 92.4±3.2°; 95.7±2.4°; 100.4 4.8° and105.6±3.3° at 70° C.

The film samples which were etched with the chromic acid solution for 30sec, 1 min and 5 min at 20° C. and for 5 sec, 10 sec, 1 min and 5 min at70° C. were then further treated with a trifunctional aziridine compound(Neocryl CX-100®), which compound prior to being used was first dilutedwith distilled water to a ratio of 1:1 at room temperature (RT) for30-40 seconds, and was then washed was distilled water for 1 minute,with methanol for 30 secds, with acetone for 30 sec and finally againwith distilled water for 3 mins, whereafter the film was dried in theabove manner. The contact angle of these modified surfaces was thenmeasured and the following values were obtained: 64.9±1.5°; 77±2.2° and71,9±2.7° at 20° C. etched films, and 70±1.9°; 68±1.9°; 70,8±1.1° and76.7 1.8° at 70° C. etched films. In order to confirm that the treatmentgave a permanent effect, the first of above-mentioned films was boiledin distilled water for 5 hours and then dried. The contact angle wasthen measured as 67.8±5.3°.

The contact angle on washed polypropylene film subsequent to treatmentwith Neocryl CX-100 without etching was found to be 98.4±2.4°.

EXAMPLE 2

The film was treated with a dichromate solution in sulphuric acid, butthis time with 2.8 percent by weight dichromate in 15.77 molar ofsulphuric acid for 30 sec, 1 min and 5 min at 20° C. and for 5 sec, 10sec, 1 min and 5 min at 70° C, whereafter the films were washed cleanwith distilled water. The films were then dried in an oven for threehours at 50° C, whereafter the water/air contact angle of the films wasmeasured and the following values obtained: 83.4±2.9°; 84.8±2,9° and83.5±2.2° at 20° C., and 84.8±1.9°; 85.3±2.6°; 93.1±2.4° and 95.4±1.5°at 70° C. The film samples were then further treated with atrifunctional aziridine compound (Neocryl CX-100), which prior to beingused was diluted with distilled water to a ratio of 1:1 at roomtemperature for 30-40 sec and then washed with distilled water for 1minute, with methanol for 30 sec, with acetone for 30 sec and finallyagain with distilled water for 3 min, and then dried as above. Thecontact angle of these modified surfaces was then measured and thefollowing values obtained: 71.0±1.7°; 72.2±1.7° and 73.3±1.4° at 20° C.etched films, and 70.5±1.5°; 72.3±1.0° and 75±0.7° at 70° C. etchedfilms. It is clearly seen that etching with the weaker dichromatesolution results in a slightly lower contact angle than when etchingwith the stronger etching solution of Example 1.

EXAMPLE 3

The films were first treated with dichromate solution in sulphuric acid(4.4 percent by weight dichromate in 15.7 molar sulphuric acid) forperiods of 30 sec, 1 min and 5 min at 20° C., whereafter the films werewashed clean with distilled water. These films were then further treatedwith 70%-nitric acid at 50° C. for 15 min, and then washed withdistilled water. The treated films were then dried in an oven at 50° C.,whereafter the water/air contact angle of the treated films wasmeasured, resulting in the following values:83±1.8°; 89.3±2.9° and79.3±3.4°. The etched films were then further treated with atrifunctional aziridine compound (Neocryl CX-100) which was used, afterbeing diluted with distilled water to a ratio of 1:1, at roomtemperature for 30-40 sec and then washed with distilled water for 1minute, with methanol for 30 sec, with acetone for 30 sec and finallyagain with dis-tilled water for 30 min, whereafter the films were driedin the aforedescribed manner.

The contact angle was then measured again, resulting in values of69.8±2.9°; 81.1±1.9° and 73.9±2.9°. In order to check the stability ofthe treated surfaces, the first of the aforementioned films was boiledin boiling water for five hours and the contact angle then measured ondry film. A contact angle of 71.5±3.1° was obtained. It will be seenfrom the results that compared with Examples 1 and 2 additional etchingwith nitric acid affords no appreciable improvements.

EXAMPLE 4

The film etched in accordance with Example 2 for 30 sec at 20° C. andtreated with Neocryl CX-100 was after-treated for 1 min with distilledwater, for 1 min with methanol and for 1 min with acetone at roomtemperature, whereafter the contact angle was measured to be 66.8±2.8°.This film was then further treated with boiling distilled water for twohours and the contact angle was measured and found to be 56.6±0.9°. Incomparison with Example 2, it will be seen that the longeraftertreatment resulted in a lower contact angle.

Thus, a still lower contact angle was obtained in comparison withExample 2, which is probably due to the effect of a third step, i.e.non-reacted functional groups from the trifunctional aziridine reactedfurther with water.

EXAMPLE 5

The film etched in accordance with Example 2 for 30 sec at 20° C. andtreated with Neocryl CX-100 was after-treated for 1 min with distilledwater and 1 min and 5 min with acetic acid at room temperature and atpH=3. Contact angles of 80.8±1.5° and 81.2±0.7° were obtained. Thetreatment with acetic acid definitely gave no improvement.

EXAMPLE 6

The film etched in accordance with Example 2 for 30 sec at 20° C. andtreated with Neocryl CX-100 was after-treated for 1 min with distilledwater and then for 1 min and 5 min respectively with lactic acid at pH=3at room temperature and at a temperature of 55° C. The following resultswere obtained:

1 min

75.5±2.4° at room temperature; 73.2±2.1° at 55° C.

5 min

68.5±1.5° at RT; 71.7±1.7° at 55° C.

The lower contact angle in comparison with Example 5 is probably due tothe presence of a hydroxyl group in the lactic acid molecule.

EXAMPLE 7

The film etched according to Example 2 for 30 sec at 20° C. and treatedwith Neocryl CX-100 was after-treated for 1 min with distilled water andthen for 5 min with malonic acid at pH=3 at RT and at 55° C.respectively. Contact angle values of 83.9±0.9° and 70.8±2.3°respectively were obtained.

The improved result, compared with Example 5, is probably becausemalonic acid has two carboxyl groups whereas acetic acid has only onecarboxyl group.

EXAMPLE 8

The film etched according to Example 2 for 30 sec at 20° C. and treatedwith Neocryl CX-100 was after-treated for 1 min with distilled water andthen for 5 min with a sodium hydroxide solution at pH=10 at RT and at55° C. respectively. Contact angle values of 59.6±2.7° and 63.4±0.8°respectively were obtained.

EXAMPLE 9

Since water reacts with the aziridine rings, a 1:1 mixture of CX-100with waterfree acetone was used to treat etched polypropene filmaccording to Example 2 for 30 sec at room temperature. The etched filmwas then after-treated for 1 min in with distilled water, 1 min withmethanol and 1 min with acetone at room temperature. The contact anglewas measured and found to be 58.4±0.9°.

The absence of water during the treatment with the aziridine compoundaccording to step 2 would seem to be preferable.

EXAMPLE 10

Instead of polypropylene film, PP-yarn was first etched with chromicacid according to Example 2 for 30 sec, and then treated with 1:1 CX-100solution in water-free acetone at room temperature for 30 sec, and thenafter-treated with methanol for 1-5 min. Since the contact angle methodcannot be used to prove the surface modifying effect on yarn, analternative method was used. Two yarn samples of mutually the samelengths, the one untreated and the other surface treated in accordancewith the afore-going, were immersed in respective water basin and thetime taken for the first water droplet to leave the basins was taken asa measurement of the surface treatment effect. The results were 4 minfor the treated yarn as against 10.5 min for the non-treated yarn,measured under equal conditions. The surface modification effect wasalso confirmed, by measuring the flow rate through the yarn when used asa stopper in the end of a funnel. The through-flow rate in the case ofthe treated yarn was much slower than that of the untreated yarn,showing that the wetting power of the yarn had been considerablyimproved by the surface modification.

EXAMPLE 11

PP-film was first etched with a chromic acid solution according toExample 2 for 30 sec at RT and then treated with a carbodiimide compound(UCARLNK XL-25 SE®) for 30 sec at room temperature. The film was thenafter-treated with methanol for 1 min and finally with acetone for 1min. The contact angle of the modified film was measured and found to be74.4±1°.

EXAMPLE 12

The PP-film etched in accordance with Example 11 and treated withcarbodiimide was after-treated for 1 min with distilled water and for 5min with methanol, whereafter the contact angle was measured and foundto be 72.2±1.8°. Since UCARLNK XL-25 SE is monofunctional, no furtherlower of the contact angle is achieved by extending the after-treatmentprocess. In order to check that a permanent surface modification hadbeen achieved, the modified sample was boiled for 2 hours in boilingacetone. The contact angle was measured and found to be 71.0±0.9°, i.e.the angle was unchanged.

EXAMPLE 13

PP-film was first etched with chromic acid solution according to Example2 for 30 sec at RT and then treated with an isocyanate compound (EstanePBA 2103®) for 30 sec at RT. The treated film was then washed withacetone for 30 sec and 1 min respectively and the contact anglemeasured. The values obtained were 80.2±1.3° and 78.2±0.5° respectively.

EXAMPLE 14

Polyethylene film (PE) of very high crystallinity (Lupolen 3741 BX®) wasfirst washed clean with acetone and then dried, whereafter the contactangle was measured and found to be 92.3±1.2°. The cleanly washed filmwas then treated with dichromate solution in sulphuric acid (2.8 percentby weight dichromate in 15.77 molar of sulphuric acid) for periods of 30sec, 1 min and 5 min at 20° C. and 70° C. respectively, and then washedclean with distilled water. The water/air contact angle was thenmeasured on the treated films, subsequent to having dried the films inan oven at 50° C. for 3 hours. The measured contact angle values were80.4±2.0°; 71.9±6.2° and 82.1±0.9° at 20° C. and 86.7±1.9°; 85.4±3.2°,and 90.8±1.2° at 70° C. The etched films were then further treated witha trifunctional aziridine compound (Neocryl CX-100) subsequent todilution with acetone to a 1:1 ratio at room temperature for 30-40 secand then washed with distilled water for 1 min, with methanol for 1 min,with acetone for 1 min and finally again with distilled water for 3 min,whereafter the films were dried in accordance with the aforegoing. Thecontact angle of these modified surface was again measured and found tobe 57.3±3.0°; 63.4±1.0° and 61.1±5.1° in the case of film etched at 20°C., and 69.3±3.4°; 74.2±2.1° and 59.8±3.4° in the case of films etchedat 70° C.

EXAMPLE 15

A PE-sample treated with a CX-100 for 30 sec as in Example 14 wasafter-treated with methanol for 1 min. The contact angle was found to be58.2±2.7°. For the purpose of determining whether or not the treatmentgave a permanent effect, the modified sample was boiled in acetone for 3hours. The contact angle remained unchanged, i.e. 57.2±0.7°.

EXAMPLE 16

The PE-film was etched in accordance with Example 14 for 30 sec and 1min respectively and then treated with a carbodiimide (UCARLNK XL-25 SE)for 30 sec at RT and then after-treated with distilled water for 1 min,with methanol for 1 min and with acetone for 1 min at RT. The film wasthen dried and the contact angle measured, this angle being found to be67.1±1.1° and 70.2±2.1°.

EXAMPLE 17

Polyester plastics produced by condensation polymerisation need not besurface oxidized in accordance with the methods before mentioned withrespect to step 1, since OH-groups or COOH-groups can be obtained byadjusting the OH/COOH-ratio during the resin manufacture.

A film sample of a polyethylene terephthalate (Mylar®) was washed cleanand the contact angle measured to be 71.2±2.2°. The film was thentreated with an aziridine compound (CX-100) for 30 sec at 20° and at 50°C. respectively and then washed with methanol for 1 min, with acetonefor 1 min and finally again with distilled water for 3 min, and thendried in accordance with the aforegoing. The contact angle was againmeasured and found to be 56±1.7° and 57.7±1°. The test showed that step2 according to the invention can be applied directly for surfacemodifying co-polymers when the presence of OH--, COOH--and C═O-groupscan be produced on the polymer surface with the aid of knowncopolymerization techniques.

EXAMPLE 18

Polytetrafluoroethylene (Algoflon®) was first washed clean with acetoneand then dried, whereafter the contact angle was measured and found tobe 120.9±9.8°. The cleanly washed film was then treated with adichromate solution in sulphuric acid (2.8 percent by weight dichromatein 15.77 molar sulphuric acid) for 30 sec and 5 min at 20° C. and 70° C.respectively, whereafter the film was washed in distilled water. Thewater/air contact angle was then measured on the treated films,subsequent to drying the film in an oven at 50° C. for 3 hours.Measurement of the contact angles resulted in values of 113.2±4.8° and122.9±1.8° at 20° C. and 118.2±3.8° and 119.1±1.5° at 70° C. The etchedfilm samples were then further treated with a trifunctional aziridinecompound (Neocryl CX-100) which was used after dilution with acetone fora ration of 1:1 at room temperature for 30-40 sec and then washed withdistilled water for 1 min with methanol, for 1 min with acetone andfinally again with distilled water for 3 in, whereafter the film waswashed in accordance with the aforegoing. The contact angle of thesemodified surfaces was then measured and found to be 105.3±1.6° and114.3±0.8° at 20° C. and 116.2±2.6° and 116.5±1.1° at 70° C.

EXAMPLE 19

Polystyrene material (PS) was also treated in accordance with theinvention. Since PS is highly sensitive to strong oxidizing agents, thedesired groups, such as --OH, --COOH or C═O on the polymer surface wereobtained by photooxidation under UV-light for 60-90 sec in the presenceof H₂ O₂. The irradiated films were then further treated with atrifunctional aziridine compound (Neocryl CX-100), subsequent todilution with acetone to a ratio of 1:1 at room temperature for 30-40sec and then washed with distilled water for 1 min, with methanol for 1min, with acetone for 1 min and finally again with distilled water for 3min, and then dried in accordance with the above. The contact angle ofthe PS-films was measured both before and after surface modification,and a lowering of the contact angle from 75.3±2.2° to 63.2±1.2° wasobtained.

EXAMPLE 20

PP-film was plasma treated in a plasma reactor (plasma-prep. 100 ,Nanotech). The film samples were placed in the reactor, which was thenevacuated to 1 torr. Oxygen gas was then streamed through the reactor ata flow rate of 20 ml/min.

The film samples were then exposed to 100 W plasma for 1 min at 60° C.

Subsequent to plasma etching, contact angles of between 91° and 67° weremeasured on different film samples, i.e. a wide spread was obtained overthe measured contact angle values. Some of the plasma etched films werethen further treated with a trifunctional aziridine compound (NeocrylXZ-100) which was used after dilution with acetone to a ratio of 1:1 atroom temperature for 30-40 sec and then washed with distilled water for1 min, with methanol for 1 min, with acetone for 1 min and finally againwith distilled water for 3 min, and then dried in accordance with theaforegoing. The contact angle of PP-films which after the plasma etchingprocess had a mean contact angle of 85.5° was measured to be 63.3±0.4°,i.e. the contact angle had been lowered by the surface treatment. Inorder to check whether or not the surface treatment effects werepermanent, the surface modified films were stored in air for some weeksand the contact angle values then measured. The contact angle value ofsolely plasma etched film returned to the original value, i.e. about90°, whereas the films which were treated with aziridine compoundsubsequent to the plasma etching process retained a contact angle valueof about 60°. This again confirms that surface treatment according tothe invention provides a permanent effect and that --OH, --COOH andC═O-groups can be provided on the polymer surface in step 1 by means ofalternative methods.

EXAMPLE 21

PP-film similar to that used in Example 20 was oxidized by means of acorona treatment. This corona treatment of the film involved exposingthe film to ionized air generated by applying a high voltage between twoelectrodes. A corona power of 2.5 kW was used.

The contact angle of film samples subjected to this corona treatment wasthen measured and found to be on average 83.1±1.2°. There was no spreadin the contact angle values obtained in this particular case. Some ofthe corona-treated films were then further treated with a trifunctionalaziridine compound (Neocryl CX-100) in accordance with step 2, which wasused after diluting with acetone to a ratio of 1:1 at room temperaturefor 30-40 sec and then washed with distilled water for 1 min, withmethanol for 1 min, with acetone for 1 min and finally again withdistilled water for 3 min, and then dried in accordance with theaforegoing. The contact angle of the treated PP-films was measured andfound to be 64.1±0.4°, i.e. the surface treatment had lowered thecontact angle. For the purpose of checking whether or not the surfacetreatment effects were permanent, the contact angle values of thesurface modified films were measured after storing the films for someweeks in air. The contact angle value of the films solely corona treatedreturned to their original value, i.e. about 90°, which is the contactangle value of PP-film prior to the corona treatment, whereas the filmswhich had been treated with aziridine compound (CX-100) after the coronatreatment retained a contact angle value of about 60°. This againconfirms that surface treatment in accordance with the inventionprovides a permanent effect and that alternative methods, of the typecorona treatment, can also be used to produce OH--, COOH--C═O-groups onthe polymer surface.

EXAMPLE 22

Instead of aziridine, isocyanate and carbodiimide compounds, thePP-films were also treated with epichlorohydrin, thiirane, azetidinone(β-lactam) and oxetane (4-methyloxetane-2-on) in step 2. The PP-filmsused in step 2 were treated in a dichromate solution according toExample 2 for 30 sec at room temperature and the contact angle wasmeasured. The contact angle on PP-films treated with chromic acid werefound to be between 82° and 85°, whereas this contact angle decreased to72°-75° subsequent to chemical treatment. Although the effects obtainedwere not as pronounced as those obtained with trifunctional aziridinecompound (CX-100), they were nevertheless quite noticeable. A check wasagain run in order to ascertain whether the effects were permanent ornot, by boiling the treated material in water and acetone respectively.No change was noticed in the contact angle values, which again showedthat the effects were permanent.

EXAMPLE 23

A polyacetal (Delrin®) was treated with chromic acid solution inaccordance with Example 2 for 30 sec in step 1 and then with aziridinecompound (CX-100) for 30 sec at RT in step 2. The treated films werethen further treated with methanol for 1 min and with acetone for 1 minat RT whereafter the films were dried in accordance with the aforegoingand the contact angle measured. A lowering in the contact angle of about12°.° wa obtained.

EXAMPLE 24

An acrylate polymer was produced by solution polymerisation of methylmethacrylate, hydroxyethylarylate, and acrylic acid in a mol ratio of0.90:0.07:0.03 in the presence of N,N'-bis-azo(isobutyronitrile) asinitiator at 60° C. This copolymer contained OH--and COOH-groups per se.

The acrylate polymer produced was reacted directly with an aziridinecompound (CX-100) for 30 sec at RT in accordance with step 2. Thetreated films were then further treated with methanol for 1 min and withacetone for 1 min at RT and were then dried in accordance with theaforegoing, whereafter the contact angle were measured. A lowering ofthe contact angle of 13.4° on average was obtained.

I claim:
 1. A method for increasing the hydrophilic properties ofpolymer surfaces of polyolefin materials, characterized by subjectingthe polyolefin material to an oxidative treatment in a first step toprovide functional groups on the polymer surface, said functional groupsbeing chemically reactable with mono- and polyfunctional aziridines andcarbodiimides; and contacting immediately after said oxidation treatmentin a second step the thus obtained polymer surface with a solution of anorganic compound, said organic compound being selected from the mono- orpolyfunctional aziridines and carbodiimides, and in a third step,followed immediately after said second step without any drying andcoating formation of said organic compound onto the polymer surface,washing the polymer surface with one or more solvents to remove everyexcess of said organic compound from the polymer surface.
 2. A methodaccording to claim 1, wherein the functional groups are selected fromthe group consisting of carboxyl groups, hydroxyl groups and carbonylgroups.
 3. A method according to claim 1, wherein the polyolefinmaterial is etched with an oxidizing acid solution in said first step.4. A method according to claim 1 wherein the polymer material in saidsecond step is treated with a polyfunctional aziridine compound.
 5. Amethod according to claim 4 wherein the polymer surface is treatedsimultaneously with or directly after said third step with one or morecompounds containing nucleophilic groups which react with stillunreacted aziridine rings by opening said rings, said compound beingselected from the group consisting of alcohols, water, amines,carboxylic acids, amino acids, hydroxycarboxylic acids and inorganichydroxides.
 6. A method according to claim 1, wherein the polymermaterial is treated with a carbodiimide compound in said second step.